Since many people think that non-nullable references can be implemented as
a library and thus don't belong to core language, I've decided to show
that it is in fact impossible to do so.
How do you enforce the following behavior:
class Foo
{
this()
{
// error: variable nonNull not initialized
}
this()
{
nonNull.someMethod(); // error: variable used before initialized
auto s = toString(); // error: can't call any methods before
initialized
nonNull = new Bar();
s = toString(); // okay
}
string toString() { return nonNull.toString(); }
NonNull!(Bar) nonNull;
}
class Bar : Foo
{
this()
{
nonNull.someMethod(); // error: variable used before initialized
super(); // initializes nonNull
nonNull.someMethod(); // fine
}
}
Without support of these use-cases NonNull!(T) is useless.
There can be other examples, but I think these are enough to prove that
non-nullable references can not be implemented in library.

Since many people think that non-nullable references can be implemented
as a library and thus don't belong to core language, I've decided to
show that it is in fact impossible to do so.
How do you enforce the following behavior:

[snip]

Without support of these use-cases NonNull!(T) is useless.
There can be other examples, but I think these are enough to prove that
non-nullable references can not be implemented in library.

Indeed. There is also the arrays I've mentioned before, and as you
mention here, fields of classes and structs. Good catch.
--
Simen

Since many people think that non-nullable references can be implemented
as a library and thus don't belong to core language, I've decided to
show that it is in fact impossible to do so.
How do you enforce the following behavior:

[snip]

Without support of these use-cases NonNull!(T) is useless.
There can be other examples, but I think these are enough to prove that
non-nullable references can not be implemented in library.

Indeed. There is also the arrays I've mentioned before, and as you
mention here, fields of classes and structs. Good catch.

Worth adding:
Even if non-null references/pointers cannot be perfectly implemented in a
library, they may still be a worthwhile addition to Phobos, to mark
function arguments and the like.
--
Simen

The above seems correct to me. You are assigning a nullable to a non-
nullable so you force the user to assess that is correct and provide an
override. Based on that I've had a crack at this myself.
http://myweb.tiscali.co.uk/adz21c/nonnull.dhttp://myweb.tiscali.co.uk/adz21c/test.d
Upto now I've only concentrated on mutable class references, but I figured I
would post what I have up to now and see what people think.
I've pulled NonNull (NN to save my fingers) construction out into a separate
function. toNN performs the runtime null check rather than the struct
constructor. This allows us to use opAssign to convert NN!Derived to
NN!Super without a null check, you only check for nulls when crossing from
the nullable world to non-nullable using toNN. It also means code like below
is not possible without explicitly saying I want to cross from nullable to
non-nullable (using toNN) which offers some sort of compile time check (not
as good as a compiler checking for "if is null" but it is something).
A a = null;
NN!A b = a; // will not compile
NN!A b = toNN(a); // inserts the null runtime check
NN!A c = b; // No null check as there is no need.
I've attempted where possible to try and keep it looking like a reference
(as you can see above). So far I have come across a few snags in my plan.
1. Default struct constructor.
This means the NN can be created without assigning a value. I have tried to
get around this issue somewhat by adding a null check in the invariant but
it seems the invariant is not called when using the default constructor.
Should it be or should it not? If it is then atleast while contracts are
enabled we get to know about uninitialised NNs.
2. opAssign and alias this on function parameters
a. For assignment I used opAssign to perform a conversion from NN!B to NN!A.
This doesn't work for function parameters of NN!A when passing NN!B so I am
forced to use toNN adding unnecessary runtime check and nasty looking code.
b. When passing NN!A to A I assume the alias this kicks in and passes
NN!A.value to A. However on function parameters this is not happening so I
am forced to manually call NN!A.value.
3. Assigning new gets checked
Below is annoying, clearly it does not need a null check.
NN!A a = toNN(new A());
I figure a function that does not do a null check *could* be offered for
this situation, but unfortunately that also destroys whatever guarantuees
you get with NN. The other idea I have had is somehow passing in the "new
A()" as an expression and evaluating if the expressions purpose is to create
an object then remove the null check, but I don't know how to do that.
4. Double checking nulls
foo(A a)
{
if (a is null) // check one
doSomething();
else
bar(toNN(a)); // check two
}
bar(NN!A a) {}
If the user checks for null before assigning nullables to NN then 2 null
checks are performed, one by the user and another by toNN. Would the
compiler be smart enough to remove the redundancy? If not then the only way
round it I can see is allow a delegate to be passed to toNN which holds the
isnull code path. Problem with this is I think it would make code quite
difficult to read. Otherwise we suck it up.
5. Consider if someone is altering code with toNN in it, the toNN will hide
a bug (if they removed a required check) till runtime that could be
identified at compile time. But I guess thats the diff between a library
version and compiler version. At least the code will die earlier at runtime
though.
Thoughts up to now? Am I barking up completely the wrong tree?

The above seems correct to me. You are assigning a nullable to a non-
nullable so you force the user to assess that is correct and provide an
override. Based on that I've had a crack at this myself.

This becomes not just an annotation, but another type. When you pass argument
by ref, you don't have to convert it to ref, you just pass it.

NN!Super without a null check, you only check for nulls when crossing from
the nullable world to non-nullable using toNN. It also means code like below
is not possible without explicitly saying I want to cross from nullable to
non-nullable (using toNN) which offers some sort of compile time check (not
as good as a compiler checking for "if is null" but it is something).
A a = null;
NN!A b = a; // will not compile
NN!A b = toNN(a); // inserts the null runtime check
NN!A c = b; // No null check as there is no need.

This is also how it is done in those languages no?
They also have null checks at every assignment, but since compiler puts
those lines, you don't know anything.
Maybe this is not what you mean?
--
Using Opera's revolutionary email client: http://www.opera.com/mail/

The above seems correct to me. You are assigning a nullable to a non-
nullable so you force the user to assess that is correct and provide an
override. Based on that I've had a crack at this myself.

This becomes not just an annotation, but another type. When you pass
argument by ref, you don't have to convert it to ref, you just pass it.

Yes, the fact it is a different type is an implementation detail of my
attempt to also get a basic NN compile-time check.

NN!Super without a null check, you only check for nulls when crossing
from the nullable world to non-nullable using toNN. It also means code
like below is not possible without explicitly saying I want to cross from
nullable to non-nullable (using toNN) which offers some sort of compile
time check (not as good as a compiler checking for "if is null" but it is
something).
A a = null;
NN!A b = a; // will not compile
NN!A b = toNN(a); // inserts the null runtime check
NN!A c = b; // No null check as there is no need.

This way or another, you need a null check. Why extra syntax?

To get informed by the compiler I am crossing into NN code that I did not
explicitly authorise. I thought that is a key part of the NN feature
request. Granted with a template I can't get it inspecting the code around
it for null checks but by making the cross from null to NN explicit it can
help people potentially catch code where they've maybe not fully assessed
the impact of the potential null floating around their code.

1. Default struct constructor.
This means the NN can be created without assigning a value. I have tried to
get around this issue somewhat by adding a null check in the invariant but
it seems the invariant is not called when using the default constructor.
Should it be or should it not? If it is then atleast while contracts are
enabled we get to know about uninitialised NNs.

There are no default constructors for structs. They use init, which is known at
compile-time, so there is no constructor for the invariant to be called after.
It's quite easy to create structs which pass their invariant when you construct
them with a constructor but whose init fails the invariant. The fact that the
invariant gets called before opAssign() doesn't help either:
http://d.puremagic.com/issues/show_bug.cgi?id=5058
The result is that at this point, you pretty much either need to have init pass
your invariant or have no invariant.
- Jonathan M Davis

Since many people think that non-nullable references can be implemented as
a library and thus don't belong to core language, I've decided to show
that it is in fact impossible to do so.
How do you enforce the following behavior:
class Foo
{
this()
{
// error: variable nonNull not initialized
}
this()
{
nonNull.someMethod(); // error: variable used before initialized
auto s = toString(); // error: can't call any methods before
initialized
nonNull = new Bar();
s = toString(); // okay
}
string toString() { return nonNull.toString(); }
NonNull!(Bar) nonNull;
}
class Bar : Foo
{
this()
{
nonNull.someMethod(); // error: variable used before initialized
super(); // initializes nonNull
nonNull.someMethod(); // fine
}
}
Without support of these use-cases NonNull!(T) is useless.
There can be other examples, but I think these are enough to prove that
non-nullable references can not be implemented in library.

Andrei's stance is, either a library addon or ship D without that feature. D's
library already contains both tuples and algebraic data types. They're simple
to use, almost like in Python. The reason for library addons isn't that builtin
features make less sense, the reason is that TDPL is already out and we can't
improve the language in any radical way.

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

People have proposed syntax that won't break existing code (e.g. T vs T).
It also means T is still nullable by default, and that's good enough for
me. Better than nothing anyway.
In addition, I really hope there will be TDPL 2nd edition some day.

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

Lets talk about solution in this thread more than politics, politics
"never" improve anything.
Also It is not only Andrei, every single people here agreed on stopping
"lets add this new feature to D today".
For the topic, i am trying to understand the use cases and implementations
in other languages first.
D was born to solve problems right?
If supporting a feature looks/is impossible for user we should improve the
language to help us solve this not just add the specific feature.
You know there'll never be enough features :)
--
Using Opera's revolutionary email client: http://www.opera.com/mail/

Also It is not only Andrei, every single people here agreed on stopping
"lets add this new feature to D today".

What's stopped is adding features to D2, but I have meant nonull types for D3.
I don't think D evolution is finished right now. On the other hand new features
add complexity, so they may refused in many cases (there are several other
things that may be added to D3, like macros, pattern matching, integral
overflows, ranged integrals, etc. But some such features add a lot of
complexity to a language that's not simple already).

If supporting a feature looks/is impossible for user we should improve the
language to help us solve this not just add the specific feature.
You know there'll never be enough features :)

Are you saying that D may be improved to allow defining good enough nonnull
types in user code? This is possible, but it's very hard. It requires macros
and a powerful type system that's probably beyond what D will ever hope to
have, because maybe not even CommonLips macros are enough for this (and even if
such changes will be implemented, the resulting language will be harde to use
as ATS or more, I don't think lot of people here will be happy with it). So to
not increase the language complexity too much, some features need to be
implemented at front-end level. It's a matter of trade-offs.
Bye,
bearophile

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

Lets talk about solution in this thread more than politics, politics
"never" improve anything.

There was this other thread here -- "why a part of d community do not
want to go to d2?"
One reason is, there's no good process for handling these feature
proposals. Walter attends useless bikeshed discussions and spreads
misinformation about things he doesn't get, Andrei has excellent
knowledge of languages but he often prefers staying in the background.
There are these DIPs in wiki4d. Were they useful? At least it seems that
this thread is leading nowhere. Half of the people don't know what non-
nullable means. It's hard to trust this process when it seems to go
nowhere. No one wants to validate the design decisions.

There are these DIPs in wiki4d. Were they useful? At least it seems that
this thread is leading nowhere. Half of the people don't know what non-
nullable means. It's hard to trust this process when it seems to go
nowhere. No one wants to validate the design decisions.

If a language feature is too much complex to understand&design for the
community of people that use the language, then it may be better to not add
that feature to the language. Maybe nonnullabile types are too much complex to
design for D. Even D2 immutability and D1 type system seem borderline to the
max complexity of things that may be added to D. The design of the module
system and immutability have some important holes still.
Bye,
bearophile

There are these DIPs in wiki4d. Were they useful? At least it seems
that this thread is leading nowhere. Half of the people don't know what
non- nullable means. It's hard to trust this process when it seems to
go nowhere. No one wants to validate the design decisions.

If a language feature is too much complex to understand&design for the
community of people that use the language, then it may be better to not
add that feature to the language. Maybe nonnullabile types are too much
complex to design for D. Even D2 immutability and D1 type system seem
borderline to the max complexity of things that may be added to D. The
design of the module system and immutability have some important holes
still.

I bet even the basic class/interface/exception system of D is too complex
to explain for some members of the audience. You can't assume that the
same people who *use* the language can/want to understand the
implementation of the features. Of course it's benefical to understand
how everything works, but in some practical tasks 1) you write some high
level code 2) compile it 3) analyze the output (the generated binary) and
4) perhaps hand optimize some parts. You don't need to understand what
the compiler actually did in all phases.

I bet even the basic class/interface/exception system of D is too complex
to explain for some members of the audience. You can't assume that the
same people who *use* the language can/want to understand the
implementation of the features.

You are right. The people that use the language&compiler don't need to
understand it fully. So are nonnull references too much hard to use? Their
syntax is simple. The limited typestate they need to be implemented well is
easy to use by the programmer. The initialization of collections is simple
enough to understand, in my opinion. So what's left is the delayed types. So in
the end I think that you are right, once well implemented the nonnullables in D
may be usable by normal programmers, despite they add some complexity to the
language.
Bye,
bearophile

I bet even the basic class/interface/exception system of D is too compl=

ex=20

to explain for some members of the audience. You can't assume that the=

=20

same people who *use* the language can/want to understand the=20
implementation of the features.

=20
You are right. The people that use the language&compiler don't need to un=

derstand it fully. So are nonnull references too much hard to use? Their sy=
ntax is simple. The limited typestate they need to be implemented well is e=
asy to use by the programmer. The initialization of collections is simple e=
nough to understand, in my opinion. So what's left is the delayed types. So=
in the end I think that you are right, once well implemented the nonnullab=
les in D may be usable by normal programmers, despite they add some complex=
ity to the language.
I'm unsure of that. For the implementation, sure; but are all wasinhg machi=
ne users specialists in fluid dynamics? The semantic side on the other hand=
seems clean to me (or, maybe I don't get it right myself).
Imagine pointers in a language where one cannnot declare vars without init.=
Then, one would either make one pointer point to an existing var, or to a =
newly allocated "target cell"; itself initialised explicitely (or by the la=
nguage's init value for that type). Something like
auto p =3D &n;
auto p =3D &(new int =3D 1);
auto p =3D &(new int); // *p =3D=3D 0
Then, you have no null ref. Or do I miss a point? An equivalent of the seco=
nd form is verboten in D, I guess, because there is no lvalue. It could be =
avoided by requiring a temp var for the target.
By the way, is it always possible to avoid pointer declarations without ini=
t, or are there cases where one must have this feature? In other words, is =
the whole null-ref feature just convenience?
Denis
-- -- -- -- -- -- --
vit esse estrany =E2=98=A3
spir.wikidot.com

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

Lets talk about solution in this thread more than politics, politics
"never" improve anything.

There was this other thread here -- "why a part of d community do not
want to go to d2?"
One reason is, there's no good process for handling these feature
proposals. Walter attends useless bikeshed discussions and spreads
misinformation about things he doesn't get, Andrei has excellent
knowledge of languages but he often prefers staying in the background.
There are these DIPs in wiki4d. Were they useful? At least it seems that
this thread is leading nowhere. Half of the people don't know what non-
nullable means.

In all honesty, the distribution of those who don't understand non-null
is about equal across the proponents and the opponents :o).

It's hard to trust this process when it seems to go
nowhere. No one wants to validate the design decisions.

This thread does have a good outcome: Walter will at least consider
improving flow analysis in constructors to support disable'd default
constructors. That is the key improvement that allows NonNull as a library.
Andrei

This thread does have a good outcome: Walter will at least consider
improving flow analysis in constructors to support disable'd default
constructors. That is the key improvement that allows NonNull as a library.

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

Lets talk about solution in this thread more than politics, politics
"never" improve anything.

There was this other thread here -- "why a part of d community do not
want to go to d2?"
One reason is, there's no good process for handling these feature
proposals. Walter attends useless bikeshed discussions and spreads
misinformation about things he doesn't get, Andrei has excellent
knowledge of languages but he often prefers staying in the background.
There are these DIPs in wiki4d. Were they useful? At least it seems that
this thread is leading nowhere. Half of the people don't know what non-
nullable means.

=20
In all honesty, the distribution of those who don't understand non-null=20
is about equal across the proponents and the opponents :o).

Andrei's stance is, either a library addon or ship D without that
feature. D's library already contains both tuples and algebraic data
types. They're simple to use, almost like in Python. The reason for
library addons isn't that builtin features make less sense, the reason
is that TDPL is already out and we can't improve the language in any
radical way.

Lets talk about solution in this thread more than politics, politics
"never" improve anything.

There was this other thread here -- "why a part of d community do not
want to go to d2?"
One reason is, there's no good process for handling these feature
proposals. Walter attends useless bikeshed discussions and spreads
misinformation about things he doesn't get, Andrei has excellent
knowledge of languages but he often prefers staying in the background.
There are these DIPs in wiki4d. Were they useful? At least it seems that
this thread is leading nowhere. Half of the people don't know what non-
nullable means.

In all honesty, the distribution of those who don't understand non-null
is about equal across the proponents and the opponents :o).